Variable capacitive elements are used in MEMS as components of microelectronic devices including sensors, actuators and resonators. With respect to sensors, such as movement sensors, micromechanical variable capacitive elements can form a component for detection because their capacitance is sensitive to changes in position. With respect to actuators microelectromechanical capacitive elements can form force transducers.
The effectiveness and performance of variable capacitive elements in a number of different applications is often a function of the magnitude of the equilibrium gap distances (widths) between fixed and movable electrodes. Further improvements in the effectiveness of such elements requires either a reduction in the gap width or an increase in electrode surface area to boost capacitance, or both.
In conventional microfabrication techniques for producing variable capacitive elements, the surfaces of electrodes in variable capacitive elements are formed directly onto a silicon structure by a lithographic patterning process followed by an etching process in which vertical edges are defined and etched into a continuous field of silicon. In these techniques, the minimum gap width and width tolerances are usually determined by lithography and etching limits. For example, epitaxial polysilicon and switched plasma etching, as developed by Robert Bosch GmbH, allows etch depths (electrode thicknesses) of 10–50 microns, and gap widths of on the order of a micron.
An article by W. T. Hsu, J. R. Clark, and L. T.-C. Nguyen, entitled “A Sub-Micron Capacitive Gap Process for Multiple-Metal-Electrode Lateral Micromechanical Resonators,” in Technical Digest, IEEE International MEMS Conference (January 2001) discusses a process for fabricating capacitive structures that combines polysilicon surface micromachining, metal electroplating, sacrificial etching, and a side-wall sacrificial-spacer technique to achieve high-aspect-ratio, submicron capacitive gaps (hereinafter referred to as “the Nguyen process”).
U.S. Pat. No. 6,249,073 to Nguyen et al. discusses a method of producing high frequency resonators using polysilicon surface micromachining technology. These processes suffer from several disadvantages, including diverse materials which may prevent use of some semiconductor fabrication equipment. Additionally, owing to the metal plating steps in the Nguyen process, capacitive elements produced according to this process will necessarily include a metal electrode (for each electrode pair). Use of a metal electrode often presents material compatibility problems. For example, MEMS-CMOS (Complementary Metal Oxide Semiconductor) integration options are limited thereby because metal makes post-MEMS-CMOS integration impossible and may limit intra-MEMS and pre-MEMS-CMOS integration. Inclusion of metals usually also precludes post-fabrication high-temperature processing. In addition, metal electrodes may experience plastic deformation or substrate adhesion failure when subject to electrostatic forces.
The Nguyen process is not amenable to forming epitaxial silicon structures. Thus, the Nguyen process may not benefit from both the high-aspect-ratio structures and reduced gap widths made possible by employing epipoly and switched-plasma etching.
Small, precise gaps may also be useful in other MEMS. For instance, high frequency MEMS resonators may require gap distances of submicron dimensions. Therefore, what is needed is a method for producing uniform narrow gaps between silicon elements.